The drive to develop bone regenerative therapies to circumvent delayed and non-union of bone fractures is an important therapeutic issue, especially considering the millions of fractures which occur annually, at least 10% of which are unable to heal by themselves. Several growth factors play key roles in the process of bone repair, and studies aimed at elucidating the factors affecting stem cell fate have recently demonstrated that the extracellular glycosaminoglycan sugar heparan sulfate (HS) plays a key role in the proliferation and differentiation of human bone marrow-derived mesenchymal stem cells (1). Osteogenic lineage fate decisions in general are known to be strongly influenced by several heparan-binding growth factors and it is widely accepted that fibroblast growth factors (FGFs) and their receptors (FGFRs) are essential to osteoblast differentiation and proliferation (2-4). Much evidence has accumulated to show that FGF cognate binding to its receptor, and thus its intracellular signaling, as is the case with a large number of other growth and adhesive factors, is in fact controlled by HS (5).
Heparan sulfate is a member of the glycosaminoglycan family of macromolecules, linear polysaccharides consisting of a repeating glucosamine/glucuronic acid disaccharide unit backbone attached to a protein core. HS has highly but variably sulfated sequences contained within its chains, organized into clusters that subtend protein binding. These sulfated structural domain motifs are primarily responsible for the regulatory properties of HS (6). HS polysaccharides display versatilities in conformation and orientation of functional groups which enable them to employ different modes of binding with any individual protein or protein complex. These selective interactions with certain proteins thus result regulation of protein activities (7). HS thus acts as a co-receptor, binding to heparin-binding growth factors such as FGF and BMP2, both protecting them from proteolytic degradation and promoting binding to their high affinity receptors (7). HS has a low affinity yet high capacity for its ligands, so drawing them onto the cell surface and their high-affinity, cognate receptors which then transduce the appropriate signal into the cells (8). Moreover, HS binding is thought to be important in providing a matrix-bound pericellular reservoir of growth factors, so promoting their long-term availability to cells (9). BMPs are also thought to be brought into register by HS with their threonine-serine kinase BMP receptors (10-12).
HS derived from bone has been shown to improve bone regeneration when added exogenously in long bone healing (13). Jackson et al. (13) applied a single dose of bone-derived HS to a rat femoral fracture at the time of injury and showed an increase in callus size and bone volume after 2 weeks. Notably, these studies used single applications of HS derived from the MC3T3-E1 preosteoblast cell line in short-term delivery devices with promising results on the early stages of healing, but triggered little difference in the later stages.
Due to the temporal pattern of growth factor expression during bone repair, studies such as Luong Van et al (16) have looked into prolonging localized delivery of HS over longer periods of healing via the sustained release of HS from polycaprolactone microcapsules.
Fibrin glue enjoys widespread clinical application as a wound sealant, a reservoir to deliver growth factors and as an aid in the placement and securing of biological implants (19-22). Several studies have shown that fibrin glue promotes the regrowth of peripheral nerves to levels comparable with standard microsurgery, making it a popular candidate for experimental nerve repair (17). In the cellular environment, fibrin specifically binds to a variety of proteins, including fibronectin, albumin, FGF2, VEGF, and IL-1 (18). These properties make it an interesting drug and cell delivery system for various applications in tissue engineering (19-23). In each case tissue regenerative capabilities were investigated with respect to tailoring the scaffold to generate certain release profiles of the growth factors over time.
Heparan Sulphate 2 was identified and described in Brickman et al. (1998), J. Biol. Chem. 273(8), 4350-4359) and was purified from embryonic day 10 (E10) murine-neuroepithelia.